[scudo][standalone] Fuchsia related changes

Summary:
Fuchsia wants to use mutexes with PI in the Scudo code, as opposed to
our own implementation. This required making `lock` & `unlock` platform
specific (as opposed to `wait` & `wake`) [code courtesy of John
Grossman].
There is an additional flag required now for mappings as well:
`ZX_VM_ALLOW_FAULTS`.

Reviewers: morehouse, mcgrathr, eugenis, vitalybuka, hctim

Reviewed By: morehouse

Subscribers: delcypher, jfb, #sanitizers, llvm-commits

Tags: #llvm, #sanitizers

Differential Revision: https://reviews.llvm.org/D63435

llvm-svn: 363705